How leopard seals lunch

Suck, swish, gulp

Eating big and little

JUST about every place on the planet has a predator at the top of the food web. The African savannahs have the lion, the jungles of Asia the tiger and the arctic tundra the polar bear. In the Antarctic it is the leopard seals who dominate as voracious hunters of penguins and other seal species. But a new study examining their feeding habits led by David Hocking of Monash University in Australia, has revealed something unusual: how leopard seals use an efficient filter-feeding system to eat near the bottom of the food chain too.

Researchers have long known that a sizeable portion of the seals’ diet is made up of krill. But exactly how they consume these tiny crustaceans has been a mystery as the seals’ mouths have large sharp teeth that seemed utterly inappropriate for capturing them. Some researchers have theorised that the seals might suck water into their mouths and swish it past their closed teeth to capture krill. But such behaviour depends upon leopard seals being able to suck up food from water, a specialised feeding behaviour that is rarely seen in top predators.

Keen to explore this, Mr Hocking turned his attention towards Casey and Sabine, two leopard seals at the Taronga Zoo in Sydney. He and his colleagues designed a clear plastic box with thin tubes in which fish could be placed. The seals could not get at them with their teeth, and instead sucked the fish right out of the tubes. Moments later, the team saw clouds of murky water and bubbles being expelled from the sides of the seals’ mouths. This, they realised, was a filter-feeding system in action.

In examining the wear on the teeth of skulls from wild leopard seals, Mr Hocking was able to work out which teeth the seals were using to filter out tiny prey. He found extensive abrasions on canines and incisors, the sharp teeth at the front of their mouths. This, Mr Hocking points out in Polar Biology, indicates that these teeth are used to chomp penguins and other seals. But since there is no such wear on the sharp and delicate postcanines, he believes that although these triangular rear teeth look fierce, when closed they form a sieve that is the perfect size for filtering. Ideal dentistry, if you like, for a mixed diet.